'\" t
.\"     Title: CREATE DOMAIN
.\"    Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets v1.75.2 <http://docbook.sf.net/>
.\"      Date: 2011-12-01
.\"    Manual: PostgreSQL 9.1.2 Documentation
.\"    Source: PostgreSQL 9.1.2
.\"  Language: English
.\"
.TH "CREATE DOMAIN" "7" "2011-12-01" "PostgreSQL 9.1.2" "PostgreSQL 9.1.2 Documentation"
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.SH "NAME"
CREATE_DOMAIN \- define a new domain
.\" CREATE DOMAIN
.SH "SYNOPSIS"
.sp
.nf
CREATE DOMAIN \fIname\fR [ AS ] \fIdata_type\fR
    [ COLLATE \fIcollation\fR ]
    [ DEFAULT \fIexpression\fR ]
    [ \fIconstraint\fR [ \&.\&.\&. ] ]

where \fIconstraint\fR is:

[ CONSTRAINT \fIconstraint_name\fR ]
{ NOT NULL | NULL | CHECK (\fIexpression\fR) }
.fi
.SH "DESCRIPTION"
.PP

CREATE DOMAIN
creates a new domain\&. A domain is essentially a data type with optional constraints (restrictions on the allowed set of values)\&. The user who defines a domain becomes its owner\&.
.PP
If a schema name is given (for example,
CREATE DOMAIN myschema\&.mydomain \&.\&.\&.) then the domain is created in the specified schema\&. Otherwise it is created in the current schema\&. The domain name must be unique among the types and domains existing in its schema\&.
.PP
Domains are useful for abstracting common constraints on fields into a single location for maintenance\&. For example, several tables might contain email address columns, all requiring the same CHECK constraint to verify the address syntax\&. Define a domain rather than setting up each table\*(Aqs constraint individually\&.
.SH "PARAMETERS"
.PP
\fIname\fR
.RS 4
The name (optionally schema\-qualified) of a domain to be created\&.
.RE
.PP
\fIdata_type\fR
.RS 4
The underlying data type of the domain\&. This can include array specifiers\&.
.RE
.PP
\fIcollation\fR
.RS 4
An optional collation for the domain\&. If no collation is specified, the underlying data type\*(Aqs default collation is used\&. The underlying type must be collatable if
COLLATE
is specified\&.
.RE
.PP
DEFAULT \fIexpression\fR
.RS 4
The
DEFAULT
clause specifies a default value for columns of the domain data type\&. The value is any variable\-free expression (but subqueries are not allowed)\&. The data type of the default expression must match the data type of the domain\&. If no default value is specified, then the default value is the null value\&.
.sp
The default expression will be used in any insert operation that does not specify a value for the column\&. If a default value is defined for a particular column, it overrides any default associated with the domain\&. In turn, the domain default overrides any default value associated with the underlying data type\&.
.RE
.PP
CONSTRAINT \fIconstraint_name\fR
.RS 4
An optional name for a constraint\&. If not specified, the system generates a name\&.
.RE
.PP
NOT NULL
.RS 4
Values of this domain are normally prevented from being null\&. However, it is still possible for a domain with this constraint to take a null value if it is assigned a matching domain type that has become null, e\&.g\&. via a LEFT OUTER JOIN, or
INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false))\&.
.RE
.PP
NULL
.RS 4
Values of this domain are allowed to be null\&. This is the default\&.
.sp
This clause is only intended for compatibility with nonstandard SQL databases\&. Its use is discouraged in new applications\&.
.RE
.PP
CHECK (\fIexpression\fR)
.RS 4
CHECK
clauses specify integrity constraints or tests which values of the domain must satisfy\&. Each constraint must be an expression producing a Boolean result\&. It should use the key word
VALUE
to refer to the value being tested\&.
.sp
Currently,
CHECK
expressions cannot contain subqueries nor refer to variables other than
VALUE\&.
.RE
.SH "EXAMPLES"
.PP
This example creates the
us_postal_code
data type and then uses the type in a table definition\&. A regular expression test is used to verify that the value looks like a valid US postal code:
.sp
.if n \{\
.RS 4
.\}
.nf
CREATE DOMAIN us_postal_code AS TEXT
CHECK(
   VALUE ~ \*(Aq^\ed{5}$\*(Aq
OR VALUE ~ \*(Aq^\ed{5}\-\ed{4}$\*(Aq
);

CREATE TABLE us_snail_addy (
  address_id SERIAL PRIMARY KEY,
  street1 TEXT NOT NULL,
  street2 TEXT,
  street3 TEXT,
  city TEXT NOT NULL,
  postal us_postal_code NOT NULL
);
.fi
.if n \{\
.RE
.\}
.SH "COMPATIBILITY"
.PP
The command
CREATE DOMAIN
conforms to the SQL standard\&.
.SH "SEE ALSO"
ALTER DOMAIN (\fBALTER_DOMAIN\fR(7)), DROP DOMAIN (\fBDROP_DOMAIN\fR(7))
